Conventional photodiodes used as a photodetector are roughly divided into a pin-photodiode (pin-PD) and a unitraveling-carrier photodiode (UTC-PD) according to their structures.
The pin-PD has a structure in which an intrinsic (i-type) optical absorption (=active) layer that is depleted in a reverse-biased state is sandwiched by a large band gap p-type electrode layer and n-type electrode layer. For a required frequency range of response, the thickness of the active region is designed and internal quantum efficiency is determined.
On the other hand, the UTC-PD has a structure in which a p-type neutral optical absorption layer that is doped beyond a predetermined concentration to prevent the depletion in the reverse-biased state and a large band-gap i layer that is depleted in the reverse-biased state are sandwiched by a p-type electrode layer and n-type electrode layer. The operation principle of the photodiode is described in Japanese patent application laid-open No. 9-275224 (1997) in more detail.
In addition, to solve a problem of a conventional photodiode in that its optical absorption layer must be thickened to improve the photoelectric conversion efficiency, and hence it cannot respond to a high-speed optical signal, Japanese patent application laid-open No. 10-233524 (1998) discloses a hybrid semiconductor photo-detector. The hybrid semiconductor photo-detector has the optical absorption layer composed of two layers, a p-type upper optical absorption layer and a high resistance n-type lower optical absorption layer, there by implementing the pin-PD structure and UTC-PD structure with a substantially single structure.
The semiconductor photo-detector with such a structure can achieve stable response speed with little variations. This is because when a specified reverse-bias is applied across the two optical absorption layers, the high resistance n-type lower optical absorption layer is depleted in its entirety to increase the drift speed of the photoexcited holes, and the p-type upper optical absorption layer provides high diffusion speed to minority electrons, which contributes to the photoelectric conversion, even if the entire p-type layer is not depleted.
However, Japanese patent application laid-open No. 10-233524 (1998) places main emphasis upon increasing the efficiency of the photodiode, and does not disclose how to design the structure of the diode for a frequency response bandwidth required. From the beginning, it has not been discussed in this technical field as to whether the structure of the diode with the two optical absorption layers is advantageous or not to increase the speed of the photodiode.
When light is launched onto the photodiode, the incident light generates electron-hole pairs in the optical absorption layer. These electrons and holes are separated in the layer, causing a current to flow through an external electronic circuit. Generally, as the optical absorption layer becomes thicker, the response speed of the photodiode is reduced because of the prolonged carrier transit time through the layer, but the active region can absorb the light more sufficiently, thereby improving the internal quantum efficiency. In other words, there is a tradeoff between the response speed and the internal quantum efficiency the most important two factors determining the performance of the photodiode, via the thickness of the optical absorption layer, and the compromise between them is primarily important.
The intrinsic response speed determined by the carrier transit speed will be described briefly. The response speed of the pin-PD is almost determined by the transit time of the holes with lower drift speed. The transit time τD of the holes approximated by neglecting the transit speed of the electrons is given by the following expression under uniform optical illumination,τD(pin)=WD/3vh.  (1)The frequency response (3-dB down bandwidth: f3dB), which is a measure of response speed, is approximated by the following expression,f3dB(pin)=1/(2πτD),  (2)where vh is the drift speed of the holes, and WD is a depletion layer width.
On the other hand, in the UTC-PD, the electron transit speed in the i-layer with the large band gap depleted in the reverse-biased state is much greater than the electron transit speed in the p-type neutral optical absorption layer. Accordingly, an effective carrier transit time τA is substantially controlled by the optical absorption layer with the slower electron transit speed, and is given by the following expression under the uniform optical illumination,τA(UTC−PD)=WA2/3De+WA/vth,  (3)assuming diffusive electron transport.In addition, the frequency response (3-dB down bandwidth: f3dB) is also determined by the diffusion current of the electrons, and is approximated by the following expression,f3dB(UTC−PD)=1/(2πτA),  (4)where De is a diffusion coefficient of the electrons, vth is a thermionic emission velocity of the electrons, and WA is the width of the p-type neutral absorption layer.
According to expressions (1)-(4), the dependence of the 3 dB bandwidth on the width of the optical absorption layer is given by the following expression for the pin-PD,f3dB(pin)∝1/WD.  (5)
As for the UTC-PD, it is given by the following expression when the term WA/vth is relatively small,f3dB(UTC−PD)∝1/WA2.  (6)Thus, the dependence of the 3 dB bandwidth on the width of the optical absorption layer differs greatly for the pin-PD and UTC-PD. Specifically, it shows an inclination that the bandwidth of the pin-PD is high in a region where the optical absorption layer is thick, and the bandwidth of the UTC-PD becomes high as the optical absorption layer becomes thinner.
To design the high-speed photodiode with increased response speed, it is advantageous to employ the UTC-PD structure. In this case, however, the optical absorption layer must be made thinner, which brings about the reduction in the internal quantum efficiency. Consequently, although the UTC-PD can achieve high speed operation, it leaves the problem of the “tradeoff between the response speed and internal quantum efficiency”, which reduces the internal quantum efficiency of the device for the high-speed operation.